1. Field of the Invention
The invention relates to an MR method utilizing a plurality of inductively coupled receiving coil systems. The invention also relates to an MR apparatus for carrying out the method.
2. Description of the Related Art
For given MR examinations it is desirable to acquire MR signals from the same spatial zone of an object by means of different, inductively coupled coils. For example, from EP-OS 271 123 it is known to form an MR image by means of a whole-body coil and on the other hand a surface coil. The image of the whole-body coil, having a low but locally uniform sensitivity, is intended to correct the image of the surface coil which has a high sensitivity which, however, is highly dependent on location. When the two coil systems are inductively coupled to one another and the MR signals are simultaneously received from the two coils, unacceptable results are obtained, for example image artifacts occur and a drastic deterioration of the signal-to-noise ratio. In one version of the known method, therefore, the MR signals, are acquired successively in time. It the coils were suitably recoupled from one another during signal reception, the described negative effects in the case of simultaneous signal reception, however, could be avoided.
The same problems occur when instead of one surface coil use is made of a receiving coil system consisting of a plurality of surface coils, for example as known from EP-OS 412 824 (FIG. 16), in which the circumstances become even more complex when the individual surface coils are also coupled to one another. The same problems may also occur when a catheter or a surgical instrument for interventional MR examinations is provided with a small signal receiving coil for the purpose of localization and when parallel therewith a globally or locally receiving second coil is used.
As has already been stated, these problems can be avoided when all inductively coupled coils do not receive the signals simultaneously but successively, and when the respective non-receiving coils are driven in such a manner that they are electrically inactive during this phase, for example by detuning or by damping. However, this procedure prolongs the measuring time required for acquiring the MR signals and changes of state of the object could then occur (motion etc.), making it difficult to relate the MR images resulting from the separate measurements to one another.